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From "Shai Erera (JIRA)" <>
Subject [jira] Commented: (LUCENE-1575) Refactoring Lucene collectors (HitCollector and extensions)
Date Wed, 01 Apr 2009 13:21:13 GMT


Shai Erera commented on LUCENE-1575:

bq. Though we now don't have a single shared ancestor class that can give you a maxScore()
when sorting by score or when sorting by fields

I don't think it's a big issue. We have just two extensions to TopDocsCollector, each tracking
maxScore differently. Perhaps later on if more extensions are created, or a demand comes from
users, we add a ScoringTopDocsCollector class?

bq. Should we implement a default TopDocsCollector.collect/setScorer/setNextReader?

I actually think that TopDocsCollector gives you exactly what I had in mind. When I started
it, there was only collect(doc, score) method, and I wanted to have a base class that will
implement getTotalHits and topDocs for you, while you only need to provide an implementation
to collect(). Now collect has really become setNextReader, setScorer and collect. Meaning,
you know what you want to do, TDC just takes care of creating the TopDocs for you, and you
can even override topDocs(start, howMany) if you want to return a different TopDocs instance.

I think now it's clean and simple.

bq. What happened to OnlyPositiveScoresFilter? (I don't see it).

Well ... you don't see it because I forgot to implement it :).
Funny thing - all the tests pass without it, meaning all that time, we were filtering <=
0 scored documents, but never really tested it ... I guess if it's because we never really
believed it'll happen?

Anyway, I'll add such a class and a suitable test class (make sure it fails w/o using that
wrapper first).

bq. Can you update javadocs eg something like "NOTE: you cannot call this method more than
once" .

That has always been the case. Previously if you called topDocs() everything has been popped
out of the queue for you. I understand what you say though, since we have the topDocs(start,
howMany) API, nothing prevents a user from calling topDocs(0, 10) and topDocs(10, 10), only
the second call will fail. However, I don't really think that's how people will use it ...
and if they do, then perhaps they should just call topDocs() and do whatever they need on
these ranges using the TopDocs object?
I'll add that to the documentation as well.

> Refactoring Lucene collectors (HitCollector and extensions)
> -----------------------------------------------------------
>                 Key: LUCENE-1575
>                 URL:
>             Project: Lucene - Java
>          Issue Type: Improvement
>          Components: Search
>            Reporter: Shai Erera
>             Fix For: 2.9
>         Attachments: LUCENE-1575.1.patch, LUCENE-1575.2.patch, LUCENE-1575.3.patch, LUCENE-1575.patch
> This issue is a result of a recent discussion we've had on the mailing list. You can
read the thread [here|].
> We have agreed to do the following refactoring:
> * Rename MultiReaderHitCollector to Collector, with the purpose that it will be the base
class for all Collector implementations.
> * Deprecate HitCollector in favor of the new Collector.
> * Introduce new methods in IndexSearcher that accept Collector, and deprecate those that
accept HitCollector.
> ** Create a final class HitCollectorWrapper, and use it in the deprecated methods in
IndexSearcher, wrapping the given HitCollector.
> ** HitCollectorWrapper will be marked deprecated, so we can remove it in 3.0, when we
remove HitCollector.
> ** It will remove any instanceof checks that currently exist in IndexSearcher code.
> * Create a new (abstract) TopDocsCollector, which will:
> ** Leave collect and setNextReader unimplemented.
> ** Introduce protected members PriorityQueue and totalHits.
> ** Introduce a single protected constructor which accepts a PriorityQueue.
> ** Implement topDocs() and getTotalHits() using the PQ and totalHits members. These can
be used as-are by extending classes, as well as be overridden.
> ** Introduce a new topDocs(start, howMany) method which will be used a convenience method
when implementing a search application which allows paging through search results. It will
also attempt to improve the memory allocation, by allocating a ScoreDoc[] of the requested
size only.
> * Change TopScoreDocCollector to extend TopDocsCollector, use the topDocs() and getTotalHits()
implementations as they are from TopDocsCollector. The class will also be made final.
> * Change TopFieldCollector to extend TopDocsCollector, and make the class final. Also
implement topDocs(start, howMany).
> * Change TopFieldDocCollector (deprecated) to extend TopDocsCollector, instead of TopScoreDocCollector.
Implement topDocs(start, howMany)
> * Review other places where HitCollector is used, such as in Scorer, deprecate those
places and use Collector instead.
> Additionally, the following proposal was made w.r.t. decoupling score from collect():
> * Change collect to accecpt only a doc Id (unbased).
> * Introduce a setScorer(Scorer) method.
> * If during collect the implementation needs the score, it can call scorer.score().
> If we do this, then we need to review all places in the code where collect(doc, score)
is called, and assert whether Scorer can be passed. Also this raises few questions:
> * What if during collect() Scorer is null? (i.e., not set) - is it even possible?
> * I noticed that many (if not all) of the collect() implementations discard the document
if its score is not greater than 0. Doesn't it mean that score is needed in collect() always?
> Open issues:
> * The name for Collector
> * TopDocsCollector was mentioned on the thread as TopResultsCollector, but that was when
we thought to call Colletor ResultsColletor. Since we decided (so far) on Collector, I think
TopDocsCollector makes sense, because of its TopDocs output.
> * Decoupling score from collect().
> I will post a patch a bit later, as this is expected to be a very large patch. I will
split it into 2: (1) code patch (2) test cases (moving to use Collector instead of HitCollector,
as well as testing the new topDocs(start, howMany) method.
> There might be even a 3rd patch which handles the setScorer thing in Collector (maybe
even a different issue?)

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